In the course of general environmental protection, increasingly stringent requirements are being made for the protection of the subsurface and ground water. Capping and lining the landfill with plastic sheets or with mineral earth substances, e.g., loam, clay-containing loam, or clay is known. If plastic sheets are applied to the embankments, they are made rough on both sides by a ribbing. When loam is applied to embankments sealed with plastic sheets, nubs must also be welded beyond the ribbing, to secure the loam against sliding down the foil, depending on the slope of the embankment.
In some circumstances, it is useful to use a combination of a mineral sealing layer and plastic covering sheets. The mineral sealing layer must, however, maintain the highest possible plasticity to avoid or at least minimize the formation of cracks in the mineral layer. Newer studies from dump excavations have shown that mineral sealing layers located under plastic sheets tend to dry out and form cracks. Crack formation is particularly a problem for mineral layers with high clay content.
For subsurfaces that are highly susceptible to settling such as old dumps, considerable improvements must be made on the subsurface to reduce the crack formation in the mineral capping layer that might be caused by uneven settling of the waste dump mass.
Certain methods can be used to reduce the uneven settling rate of shifting subsoils. One method includes intensive depth compression with gravel piles. Another method is high shear-strength support layers arranged under the mineral capping layer to achieve the most uniform settling behavior in the region of application. Despite these measures, settling cracks cannot be completely avoided since the settling and sinking conditions in an old dump mass are independent of the load due to different types of deposits within the dump. Added to the problem of shifting dump masses are volumetric changes due to degradation caused by rotting and the shrinkage caused by drying. The same holds true for settleable subsurface when high loads are applied to the capping layer or layers.
For the capping layer to have sufficient protection against percolating surface water and for liners against water from within the waste dump it is required to form the sealing layer with a minimum gradient of 3 to 5% with due consideration for the settling and sinking so that trapped water can readily run off. High rates of settling or widely different rates of uneven settling may require considerably greater gradients than is normally required thereby increasing the cost of applying the appropriate grade to the landfill.
For the surface or covering layers a continuous surface filter such as gravel, coarse sand, or filter mats may be used over the mineral sealing layer to drain the seepage from within the landfill or the surface water from the top soil or cultivable soil applied over any capping layer. If there are long periods of dry weather, the capping layer under the cultivable soil and the surface filter might become so dried out that shrinkage cracks will form. This is particularly a problem for cover layers of clay, loam, or other artificial mixtures, e.g., tailings with additives of flotation foam and bentonite or mineral mixtures with additives of bentonite. The drying effects in combination with uneven settling rates present the possibility of a high rate of crack formation which can lead to complete destruction of seal integrity so that the waste dump is no longer properly functional.
Due in part to heightened environmental awareness and a few isolated instances of problems, regulatory agencies have raised the requirements for cover layers of waste dumps and have made them equivalent to the quality requirements for liner layers. With the conventional mineral capping materials, the more stringent requirements translate into greater layer thicknesses with the correspondingly higher costs.
From earth works and foundation construction it is known to apply geotextile fabrics, i.e., water permeable flow material or fabric under dams on soft soil layers. These geotextile fabrics provide a uniform surface that is interconnected to hinder the partial sinking of the dam banking material into the soft subsurface and prevents localized fractures in the embankments and a great measure of the adverse effects from strongly differentiated settling conditions. Further applications of geotextiles are described in "Merkblatt fur die Anwendung von Geotextilien im Erdbau", 1987 Edition, Forschungsgesellschaft far StraBen- und Verkehrswesen, Arbeitsgruppe Erd- und Grundbau, Alfred-Schutte Allee 10, 5000 Koln 21. The use of geotextiles for separation and protective layers is also known in waste dump construction.
Despite the above described measures, it has still not been possible to produce sealings and in particular cover sealings for waste dumps, so that they remain functional over longer duration despite the more or less unavoidable crack formation.
It would be desirable to have a cap construction and/or a base or liner construction that would maintain a high level of seal integrity despite uneven settling or extended periods of dry weather.
It would be desirable if the cap, base, or liner construction would compensate for the adverse effects on seal integrity caused by the formation of cracks in such layers.